Mis-Matched Battery Question

[clubhouse]

A buddy and I where talking recently, he is about to add a 2nd Group 24 battery to his TT. His existing Group 24 is about 9-10 Mths old and he was simply going to buy a 2nd to add to the exisiting, cheaper than replacing both he said. I told him that I always heard that batteries banks should be purchased and reaplaced together, so that all the batteries are the same type, about the same age and capability.

He didn't think it mattered, and I wasn't sure why it did so thats my question to you DC experts out there.

-- Why do you need to purchase/replace battery banks together?
-- What will happen if you simply add a 2nd battery of the same Grp size but differnt age and mfg to an existing battery?

Look forward to the education.

Thanks in advance

[David472]

Every battery has a certain amount of internal resistance. As the battery ages, that resistance value increases a small amount each year. Eventually it gets too high, and the battery has lost too much capacity. For two batteries in parallel, the stronger one will get dragged down by the weaker one, and the overall capacity is no better than the weaker one times 2. If his "old" battery is 10 months old, and has been well cared for, he will likely be just fine doing that, provided his new one is the same make and model. I would add, the "new condition" resistance value is dependant on the physical construction inside the battery, and each manufacturer has a slightly different build structure. That's why it's best to have the pair be from the same manufacturer. For critical applications, some people will go the effort to measure a bunch of new batteries from the same manufacturer for internal resistance, and select a pair which are a close a possible to each other for the internal resistance.

[clubhouse]

Thank you, that makes a lot more sense now.

[RoyBraddy]

For what ever it is worth - I originally started out with two each GP24 85AH 12VDC Interstate Batteries that came with my POPUP in 2008. In 2009 I added two more identical 85AH Interstate 12VDC batteries to increase my battery bank capacity. I also introduced smart-mode charging converter/charger at this time and for the most part this trailer has been on-charge continuous sitting in my backyard setup when not on the road.

I was able to use all four batteries in my parallel setup using Blue Sea 9001E battery switches while I was experimenting with what to do with my 340AH battery capacity for camping off the power grid.

I ended with larger battery cables, additional 9001E battery switches, two Inverters (600W PSW and a 1500W PSW) hard wired to my battery switches, and was doing rather well re-charging my two 85AH battery banks using my 2KW Honda generator in a 2-3 hour generator run time on a daily basis camping off the power grid.

This worked pretty good for about one year and in 2010 I did lose one of my newer 85AH Interstate batteries due to a shorted cell. I kept close watch on battery fluid levels and never ran any of the batteries below 11.9VDC - 11.5VDC when camping off the power grid. Over the winter months in 2010 one of the newer 85AH batteries did develop a shorted cell and had to be removed from the battery bank 2. I continued on with my original two 85AH Interstate batteries in BANK 1 and only one 85AH Interstate battery in BANK 2 (255AH capacity total). In march of this year (2012) I lost the second newer 85AH Interstate Battery due to a shorted cell so I am now down to the one 170AH Battery Bank 1 which is still using the original 2008 85AH Interstate batteries. Each Battery Bank 1 and 2 are wired identical with each of the NEGATIVE battery terminal going directly to frame ground using equal lengths 4-AWG cable. I use a four position battery switch feeding each POSITIVE battery terminal with same length battery cables. Each battery bank can be switched OFF-BAT1-BOTH-BAT2 using my four position battery switches. I use a third master Battery Switch 9001E to select OFF-BANK1-BOTH-BANK2 using same length battery cables. My PD9260C is metered and typically feeds 52-54 AMPS measured at the input of the MASTER four position switch when being charged in 14.4VDC boost mode. I never measured what each battery was being fed by the PD9260C Converter. All battery cables associated with my two battery banks are 4AWG Ancor brand battery cables. My 12VDC battery monitor meter setup measured each battery bank at the input of the battery bank four-position battery switch. This is where I monitored the re-charge DC voltage of around 12.6VDC to 12.7VDC after each bank would settle down after being charged at 14.4VDC for 2-3 hours as well as the discharged DC voltage of 11.9VDc to 11.5VDC discharged state when being used throughout the one day/night battery run. Each battery bank was being monitored by a separate 12VDC digital voltmeter whereas the system current drain was only monitored by a single DC ampmeter in the main line feeding the 12VDC Distribution Panel cable.

All of this was with minimal battery maintenance and I got somewhat complacent on checking battery fluid levels so I don't know if I could have saved the 2012 battery failure or not.

I suspect the remaining two each 85AH Interstate batteries will not make it through this year (2012) and have started looking around for replacement batteries at a good price and maybe a re-design of my two battery banks.

I am wanting to purchase four each Trojan T-1275 12VDC 150AH batteries and install two separate battery banks, one being located in the truck bed area of my truck and the other bank being on the tongue of the OFF-ROAD Trailer. The T-1275AH batteries weigh in at 82LBS each.

I don't know if adding the second bank of 85AH Interstate batteries had anything to do with this failure or not. I guess I really wasn't mixing any new battery with an older battery in the same bank but all four was being serviced with the same PD9260C Converter/charger with my Blue Sea battery switches providing equal length cables for all four batteries. It was interesting to me that the two newer 85AH Interstate batteries failed first.

I suspect four to five years from the cheap 85AH Interstate Batteries is probably what one should expect.